This invention relates to improved multilayer CVD diamond films and methods for making same. More particularly, the present invention relates to multilayer CVD diamond film wherein diamond nucleation sites comprised of metal are positioned between the diamond layers. This is accomplished by interrupting the growth of CVD diamond and depositing a metal to serve as a new diamond nucleation site.
Recently, industrial effort directed toward the grown of diamond at low pressures, where it is metastable, has increased dramatically. Although the ability to produce diamond by low pressure synthesis techniques has been known for decades, drawbacks, including extremely low growth rates, prevented wide commercial acceptance. Recent developments have led to higher growth rates, thus spurring recent industrial interest in the field. Additionally, the discovery of an entirely new class of solids, known as "diamond-like" carbons and hydrocarbons, is an outgrowth of such recent work.
Low pressure growth of diamond has been dubbed "chemical vapor deposition" or "CVD" in the field. Two predominant CVD techniques have found favor in the literature. One of these techniques involves the use of a dilute mixture of hydrocarbon gas (typically methane) and hydrogen, wherein the hydrocarbon content usually is varied from about 0.1% to 2.5% of the total volumetric flow. The gas is introduced via a tube located just above a hot tungsten filament which is electrically heated to a temperature ranging from between about 1750.degree.-2400.degree. C. The gas mixture disassociates at the filament surface, and diamonds are condensed onto a heated substrate placed just below the hot tungsten filament. The substrate is held in a resistance heated boat (often molybdenum) and heated to a temperature in the region of about 500.degree.-1100.degree. C.
Reference is made to U.S. Pat. No. 4,434,188, which describes a CVD process of causing diamond nucleation and growth from a heated gas mixture in contact with a substrate.
The second technique involves the imposition of a plasma discharge to the foregoing filament process. The plasma discharge serves to increase nucleation density and growth rate, and it is believed to enhance formation of diamond films, as opposed to discrete diamond particles. Of the plasma systems that have been utilized in this area, there are three basic systems. One is a microwave plasma system, the second is an RF (inductively or capacitively coupled) plasma system, and the third is a d.c. plasma system. The RF and microwave plasma systems utilize relatively complex and expensive equipment, which usually requires complex tuning or matching networks to electrically couple electrical energy to the generated plasma. Additionally, the diamond growth rate offered by these two systems can be quite modest.
Reference is made to application Ser. No. 06/944,729 of Anthony et al., filed Dec. 22, 1986 now abandoned, assigned to the same assignee as the present invention, which discloses a method and apparatus by which diamond crystals are caused to nucleate and grow on a substrate by means of a heated filament and luminescent gas plasma activated hydrogen-hydrocarbon gas mixture. The gas mixture is subjected to concurrent activation by an incandescent tungsten wire electrical resistance heater and by electromagnetic microwave energy to become a luminescent gas plasma with a significant atomic hydrogen content. The activated gas mixture is brought into contact with a heated substrate, and, as a consequence thereof, diamond crystals are formed or nucleated on the substrate from the activated gas mixture, which is followed by diamond crystal growth.
In general, processes for the chemical vapor deposition of diamond involve the selection of operating parameters, such as the selection of a precursor gas and diluent gases; the mixture proportions of the gases; gas temperature and pressure; substrate temperature; and means of gas activation. These parameters are adjusted to provide diamond nucleation and growth on a substrate. Parameters which affect diamond nucleation and growth have been investigated. For example, it is known that the CVD diamond tends to nucleate on certain materials more readily than others such as silicon and its compounds.
It is also known that if the hydrocarbon concentration (methane) is increased above 3% , graphite deposition becomes more evident.
The surface chemistry of diamond growth has also been investigated. For example, it is well known in the art of CVD diamond that atomic hydrogen is necessary to stabilize the surface of diamond film by terminating the carbon atoms with hydrogen and preserving the sp3 bonding of carbon. If the supply of atomic hydrogen is not present, the surface of the diamond film will become more complex, forming other types of bonding such as sp2 and sp.
CVD diamond films grow epitaxially using conventional methods.
This allows the grain boundaries between individual crystals to lie in a plane through the thickness of the diamond layer. These boundaries are regions of lower strength; therefore, cracks can propagate more easily through them. Since there is no interruption of the grain boundaries through the thickness of the layer, layers tend to crack and fall apart. If the grain boundaries were interrupted and the diamond film multilayered, crack propagation would be more difficult in such a film, and the structure would give the film more transverse strength.
Methods wherein the growth of diamond is interrupted or growth conditions are varied may provide some interruption of the grain boundaries. These processes typically have other objectives. For example, diamond growth is interrupted in some processes to remove graphite and the hydrocarbon/hydrogen ratio in the gas mixture is varied in some processes to maintain fine grains. However, by simply interrupting or altering the growth phase, it is possible for the new diamond growth to maintain the same grain boundaries. Where there is some disruption of the grain boundaries and multilayered films are formed, there may be weaknesses in the film if upon renucleation some of the grain boundaries are maintained. It is desirable to provide a multilayer diamond film of reliable strength which is greater than conventional CVD diamond films.